1. Field of the Invention
The present invention relates to a process for forming a coating on a localized area of a superalloy component, especially such components intended for use in aeronautical applications such as aircraft engines. The invention relates also to the coated component thus obtained, as well as to the composition of the three layer deposit used in forming the coating.
2. Summary of the Prior Art
It is known in numerous applications to improve the service life of components by applying a coating to a localized area of the component so as to improve the surface properties in that area depending on the specific stresses or contacts to be encountered. Examples of surface treatment techniques of this type are described in FR-A-2 397 259, which proposes the deposition by fusion welding of a coating of an alloy which withstands cracking at the tip of a blade, followed by a layer of an alloy which is hard and/or resistant to oxidation corrosion. Also known from FR-A-2 511 908 is a diffusion brazing process involving the application to a nickel or cobalt based superalloy part of an elementary part in the form of a presintered blank made from a mixture of two powders, one of which, termed the coating layer powder, is present between 5 and 25% by weight of the mixture and comprises a base of nickel, chromium and boron, or nickel, cobalt, silicon and boron. Furthermore, U.S. Pat. No. 4, 705,203 discloses a process for making good the surface defects of superalloy components involving the plasma flame spraying of two successive layers of different compositions, then heat treatment during which only the first layer is melted and the surface layer is then obtained.
The manufacturing techniques described in FR-A-2 511 908 require, in particular, the use of a homogeneous mixture of powders to prepare a self-brazable sintered material for application by brazing to form a coating on a localized area of a superalloy component. In this case, the maximum temperature at which the superalloy component may be used must remain substantially lower than the brazing temperature.
One of the aims of the invention is to produce, by brazing, a coating on at least one localized area of a superalloy component which will permit the superalloy component to be used at a temperature equal to, or greater than, the brazing temperature. This problem stems in particular from the use of some superalloy which do not tolerate a heat treatment cycle at a temperature greater than their normal temperature of use, and is encountered especially in certain aeronautical applications such as aircraft engine components made of monocrystalline alloys, one example of which is given in EP-A-149 942. No fully satisfactory solution to this problem has yet been proposed in the art.